Optimistic Neurons

In rats given a choice between two rewards, neurons that predict the value of expected rewards responded as though the animal had chosen the best available reward, no matter what the rat actually did. These results, reported online in Nature Neuroscience, suggest that the opportunity to make a choice may be as valuable as the best available option on the menu.

The authors trained rats to learn to associate each of three odours with an action. Two of the odours signalled that the rat was required to move either left or right to receive a reward, with one location containing a better reward than the other. The third odour indicated that the animal could choose to go to either location. Dopaminergic neurons in the ventral tegmental area, part of the brain's 'reward processing system,' responded more for the odour that cued the rat to move to the location associated with the better reward, as expected from previous studies.

The rewards varied either in the amount of juice or in how long the rat had to wait for its delivery, and responses to the odour cues were correlated with both reward size and delay. The dopaminergic neurons responded just as strongly to the odour indicating that the rat could choose which reward to collect as to the better reward, even though the rat eventually chose the worse reward nearly 30% of the time on the free choice trials.